There's a lot of them out there, but they all have their assorted quirks, making different ones better for different things.
For example, last.fm doesn't allow pausing, Pandora is US-only, and so on.
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I've been using Amazon because their MP3s are drm-free, but now I'm curious are there any music-resellers that have distribute drm-free, and have a decent policy that in the event of data-loss I can ...